Quickly flashing green LED indicates the screen service restarting. Sometimes it is intentional, like when you e.g. toggle a lockscreen patch in Patchmanager.

I don't use the media control patch but I have encountered the unexpected flashing green LED quite a lot. Sometimes, when the screen comes back, my background image is gone. It is always triggered by trying to make the lockscreen do "something" (e.g. just moving it up and down) after applying some patch. Your double tap may fall in this category.

I have had a look at some of the lockscreen-modifying patches but I did not see anything that might be responsible for this behaviour. It looks like there is some inherent bug in Sailfish and the patches may expose it. Again, I did not see anything in the patches that would do something hugely different from what the lockscreen does anyway so I suspect the patches expose it by merely touching the lockscreen. It looks like the part of Sailfish that handles the lockscreen is not particularly stable.

The fact that it seemingly affects some users but not others may of course be due to some people not noticing it and others denying it exists. But it is even more likely that there is some edge condition that triggers for some configurations but for others. I think we need to investigate it better and come back to Jolla with a detailed bug report and steps to reproduce.